doc: replace "practise" with "practice" in Documentation

To keep the Documentation consistent either
"practise" or "practice" should be used.

Since there are 3 lines with "practise"

  ~/linux/Documentation$ grep -r practise * | wc -l
  3

and 108 lines with "practice"

  ~/linux/Documentation$ grep -r practice * | wc -l
  108

this patch converts "practise" to "practice".

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>